mp3

UK: /ˌɛmpiːˈθriː/

WordReference Random House Unabridged Dictionary of American English © 2025

MP3,¹ 

1. Computing the file extension for MPEG Audio Layer-3, a set of standards for compressing and downloading audio files from the Internet. Cf. MPEG 
• 1990–95

Collins Concise English Dictionary © HarperCollins Publishers::

MP3 abbreviation

1. MPEG-1 Audio Layer-3: software that enables files to be compressed quickly to 10% or less of their original size for storage on disk or hard drive or esp for transfer over the internet
2. an audio or video file created in this way